Packaging steel is generally provided as single or double reduced tin mill products in thicknesses between 0.14 and 0.49 mm. A Single Reduced (SR) tin mill product is cold rolled directly to the finished gauge and then recrystallisation annealed. Recrystallisation is brought about by continuous annealing or batch annealing the cold rolled material. After annealing the material is usually temper rolled, typically by applying a thickness reduction of 1-2%, to improve the properties of the material. A Double Reduced (DR) tin mill product is given a first cold reduction to reach an intermediate gauge, recrystallisation annealed and then given another cold reduction to the final gauge. The resulting DR product is stiffer, harder, and stronger than SR, allowing customers to utilise lighter gauge steel in their application. These uncoated, cold rolled, recrystallisation annealed and optionally temper-rolled SR and DR packaging steels are referred to as blackplate. The first and second cold reduction may be given in the form of a cold rolling reduction in a cold-rolling tandem mill usually comprising a plurality of (usually 4 or 5) rolling stands.
Recovery Annealing (RA) is a cost effective method to produce packaging steels at a strength level comparable to Double Reduced (DR) grades, but with a better formability and notably elongation values. There are three stages in the annealing process, with the first being the recovery phase, which results in softening of the metal through a partial removal of crystal defects (the primary type of which are dislocations) and the decrease of the stored energy introduced during the deformation. Recovery phase covers all annealing phenomena that occur before the appearance of new strain-free grains. The second phase is recrystallisation, where new grains with high angle boundary nucleate and grow at the expense of grains with high stored energy. This is to be avoided in the process and product according to the invention because it leads to a sudden and dramatic decrease of the tensile properties and increase of the elongation values. The third stage is grain growth after recrystallisation.
Unfortunately a drawback of RA is that mechanical properties are usually non-homogeneous along the strip length and also from strip to strip. These variations in mechanical properties are the consequences of too dynamic a response of the grade and/or fluctuations of the temperature of the annealing furnace.